I believe you need to check your normals in blender. In edit mode, make sure they are facing the right direction.
You can turn on viewing of their direction by clicking n to bring up the menu and then turn on the normal so you can see them. If you increase the size (0.80 below) they really stand out. The way the line points is the way the normal or face will be viewed. If you view the backside of a face you simply look right through it. There is an option to recalculate normal under Mesh/Normals or select a face that is incorrect and do flip normal if it is wrong.
abow.
I seen a lot of question but when I made the walls opposite I see through the outside. So what is the fix for this problem?
That’s only part of the problem. . What he is saying is he flipped the normals already. So the problem here is a plane will only Render on one side. You can do two things. In the details panel of the mesh editor you can select 2sided and this will render the planes on both sides. Another option is to use boxes for your walls and make sure the normals inside the box point in and the ones outside the box point out. This second option is best because you can put one material or texture on the outside and a different set of textures on the inside.
For texturing an object you can assign multiple material slots in blender to different faces of a mesh. Unreal will see these and give multiple slots in the editor for you to drop your materials on. Cheers!